Allow Makefile accept env variables (for compatibility with rpm and deb package builders)
Allow Makefile accept env variables (for compatibility with rpm and deb package builders)

DESTDIR is the root by default
PREFIX is /usr/local by default (may be different or some OSes)

Elastica/mongo-c-driver-legacy,Elastica/mongo-c-driver-legacy,HeliumProject/mongo-c,HeliumProject/mongo-c,mongodb/mongo-c-driver-legacy,Elastica/mongo-c-driver-legacy,Elastica/mongo-c-driver-legacy,HeliumProject/mongo-c,mongodb/mongo-c-driver-legacy,mongodb/mongo-c-driver-legacy,mongodb/mongo-c-driver-legacy,HeliumProject/mongo-c